An attempt is being made in the model sensory system Phycomyces to dissect the metabolic pathway leading from photoreception to the growth output. Cyclic AMP, cyclic GMP and associated enzymes are used as tools to probe the mediation of transduction. Both cyclic nucleotides respond in seconds to light stimuli in a graded fashion. Mutants with higher behavioral thresholds also have the same cyclic AMP threshold but all maintain a cyclic GMP response even below this light intensity. Phosphodiesterase has been well characterized and is activated by flavins and by blue light. Adenylate cyclase is activated by catecholamines and by blue light. The latter is located in the perceptive growing zone only. Certain mutants have abnormal enzymes. Phycomyces possesses an olfactory response to many volatile chemicals which can be correlated with human olfaction.